Even in their heyday, dinosaurs were not quite as dominant as popular myth makes them out to be

ONE answer to the question, “What ate dinosaurs?” is, obviously, “Other dinosaurs.” Theropod predators like Tyrannosaurus and Allosaurus loom large in the imagination of every lover of prehistoric monsters, and their animatronic fights with the likes of Diplodocus and Stegosaurus are the stuff of cliché. Science, though, tries to look beyond the obvious, and at this year's meeting of the Society of Vertebrate Palaeontology, held in Las Vegas, some of the speakers asked whether the top predators of the Mesozoic era really were all dinosaurs. Their conclusion was “no”. Another group of reptiles, until recently neglected, were also important carnivores. And it is a group that is still around today: the crocodiles.

That the past role of crocodiles (or, strictly, crocodilians, since they came in many sizes and shapes, not all of which resemble the modern animals) has been underestimated was suggested a few years ago by Paul Sereno. Dr Sereno, a palaeontologist at the University of Chicago, uncovered a crocodile-dominated ecosystem from about 100m years ago (the middle of the Cretaceous period), in what is now north Africa. Besides water-dwelling giants similar to (though much bigger than) today's animals, he found a range of forms including vegetarians and species that ran on elongated legs—more like dogs than crocodiles. That discovery has prompted other fossil hunters to look elsewhere. As a result, even the well-studied rocks of North America are revealing that dinosaurs did not have it all their own way in the ecosystems of the Mesozoic—as Stephanie Drumheller of the University of Iowa and Clint Boyd of the University of Texas at Austin explained to the meeting.

The Cretaceous equivalent of zebra and antelopes—the victim species in every wildlife documentary about the dramas of the African savannah—were herbivorous dinosaurs called ornithopods. Frequently, these were taken by theropods. But not always. When Ms Drumheller and Mr Boyd examined the bones of juvenile upper-Cretaceous ornithopods dug up in Utah they saw marks on one skeleton that looked suspiciously like those modern crocodiles inflict when biting and tearing at their prey. On examining these marks more closely, they found a crocodilian tooth stuck in one of them.

Crocodile tears

It was not a large tooth. Its size suggests the animal which made it was no more than a metre and a half (about 5 feet) long. Such a predator would have been unable to take on an adult ornithopod. Nevertheless, this tooth is the first unarguable proof that crocodilians did indeed snack on dinosaurs. Moreover, it helps to confirm suspicions that the other crocodile-bite-like marks that Ms Drumheller and Mr Boyd have discovered really are what they look like. By combining that with an analysis of the whole site, the two researchers argue that what they have discovered is a dinosaur nesting ground that was being raided by crocodilians.

Such suspicions have been aroused before. Other sites in Utah are known to be dinosaur nesting grounds, since eggs are found there. Crocodilian bones frequently turn up at such sites. Ms Drumheller and Mr Boyd, however, seem to have nailed the connection down. Juvenile dinosaurs, at least, were indeed the prey of crocodilians. But what about adults?

More than mere morsels

To investigate that question, Martin Lockley at the University of Colorado, Denver, and Spencer Lucas of the New Mexico Museum of Natural History and Science, turned to one of the most famous fossil phenomena on the planet—the dinosaur freeway that runs through Colorado, New Mexico, Kansas and Oklahoma. This collection of tracks, scattered over several sites of the same age along the coast of an inland sea, is thought to mark an ancient migration route. The traces of more than 1,380 individual animals can be distinguished. Most, but not all, were ornithopods. Some were small carnivorous dinosaurs—the sort that might pick off young stragglers in the way that the crocodilians identified by Ms Drumheller and Mr Boyd did. But there is, Dr Lockley and Dr Lucas realised, something missing from the picture. When they looked for traces of big predatory dinosaurs, they found none.

That is ecologically absurd. Unless, of course, the top predator of the system—the one that could hunt down adult ornithopods—was not a dinosaur at all. And, when Dr Lockley and Dr Lucas re-examined the tracks they found that that was exactly what was going on. Instead of theropod footmarks, they found those of crocodilians. More than a quarter of the places where the dinosaur freeway surfaces have yielded signs of crocs. And they were big: sometimes more than four metres long. That is certainly large enough to take on an adult ornithopod.

Such megacrocs, then, could easily have acted as top predators in this ecosystem. But that does not completely explain the absence of theropod tracks. Modern migrating herbivores fall victim to many sorts of carnivore: big cats, wolves and hyenas, to name but three. The marshy conditions of the dinosaur freeway (the reason its footprints formed, and have survived) may, though, have favoured crocodilians over predators that had evolved on drier land. In that sort of environment even a big theropod would constantly have been looking over its shoulder. Perhaps the real reason why they did not plant their footprints on the dinosaur freeway is that they might have ended up as prey, as well.